Fiber reinforced ceramic matrix composite internal combustion engine intake/exhaust valves

Abstract

A fracture-resistant, thermally stable intake or exhaust valve for an internal combustion (IC) engine. The valve has a stem portion and a head portion, both of which are formed of fiber reinforced ceramic matrix composite (FRCMC) material. This FRCMC material generally includes a polymer-derived ceramic resin in its ceramic state, fibers, and filler materials. Employing FRCMC material to form the valve is advantageous as FRCMC material is highly temperature resistant and temperature stable, thereby allowing for increased engine operating temperatures. FRCMC material is also ductile, thus making the valve fracture resistant. The FRCMC material is also flaw-insensitive in that any flaw within the structure of the valve will not result in cracking and failure. In addition, FRCMC valves are considerably lighter than the existing metal valves. This provides an opportunity to reduce the weight of the overall valve train, thereby increasing engine performance. The addition of certain filler materials allows the FRCMC material to be tailored to exhibit a desired degree of harness and/or a desired coefficient of friction. Increasing the hardness of the head of the valve allows it to better withstand the violent environment of the engine cylinder, and lowering the coefficient of friction facilitates the valve movement within the engine.

Wherefore, what is claimed is:  
     
       1. A method for forming a stem and a head portions of a fracture-resistant, thermally stable valve from fiber reinforced ceramic matrix composite (FRCMC) material for an internal combustion engine, the method comprising: 
       (a) placing a mixture comprising pre-ceramic resin and fibers into a female die of a mold;  
       (b) pressing a male die of the mold onto the female die so as to mold the valve in the cavity formed between the female and male dies, said cavity having a shape corresponding that of the valve;  
       (c) heating the mold at a temperature and for a time associated with the pre-ceramic resin which polymerizes the resin to form a fiber-reinforced polymer composite valve;  
       (d) removing the polymerized composite valve from the mold;  
       (e) firing the polymerized composite valve at a temperature and for a time associated with the polymerized resin which pyrolizes.  
     
     
       2. The method of  claim 1 , wherein the step of placing a mixture comprising pre-ceramic resin and fibers into a cavity of a female die of a mold comprises the steps of: 
       (a) saturating a woven fiber rope with pre-ceramic resin;  
       (b) placing the resin-saturated fiber rope into a portion of the a female die of the mold in which the stem portion of the valve is to be formed; and  
       (c) placing a quantity of FRCMC bulk molding compound into a portion of the female die of a mold in which the head portion of the valve is to be formed, said FRCMC bulk molding compound comprising pre-ceramic resin and fibers.  
     
     
       3. The method of  claim 2 , wherein an end of the woven fiber rope is frayed, and wherein the step of placing the resin-saturated fiber rope into the portion of the female die of the mold in which the stem portion of the valve is to be formed comprises placing the frayed end of the fiber rope into the portion of the female die in which the head portion of the valve is to be formed. 
     
     
       4. The method of  claim 2 , wherein the FRCMC bulk molding compound further comprises filler material which increases the hardness and decreases the coefficient of friction of the head portion of the valve, said filler material being of sufficient quantity to produce a desired degree of hardness and a desired coefficient of friction. 
     
     
       5. The method of  claim 2 , wherein prior to step (b), the exterior of the resin-saturated fiber rope is coated with filler material which decreases the coefficient of friction of the surface of the stem portion of the valve, said filler material being of sufficient quantity to produce a desired coefficient of friction. 
     
     
       6. The method of  claim 1 , wherein the fibers are coated with an interface material comprising comprises at least one 0.1-0.5 micron thick layer of at least one of carbon, silicon nitride, silicon carbide, silicon carboxide, or boron nitride. 
     
     
       7. The method of  claim 1 , further comprising upon the completion of step (e), the steps of: 
       (f) immersing the valve containing pores formed during firing into a bath of a pre-ceramic resin to fill the pores;  
       (g) heating the valve at a temperature and for a time associated with the resin filling said pores so as to transform it to a ceramic material;  
       (h) repeating steps (f) and (g) until the pore density within the valve is less than a prescribed percentage by volume.